Method to solids-pack non-vertical wellbores

ABSTRACT

A method of preparing a solids-pack for use in oilfield completion operations involves staged pumping using two distinct types of carrier fluid with distinct solids loadings during the solids deposition process. The method can be employed in packing of non-vertical oilfield wellbores for formation stabilization, fracture stabilization and/or sand control, particularly around sand exclusion devices. In preferred embodiments the method results in greater uniformity of packing, and also offers the advantage of much more rapid deposition and therefore of completion of the pack phase, reducing overall rig time.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to the field of oil or gas wellcompletions. More particularly, it relates to the field of sand controland solids-packing of wellbores, and to the use of media for carryinggravels, proppants and other solids into the wellbore to a pack siteduring well completion operations.

[0003] 2. Background Art

[0004] It is well-known that solid particles, such as gravels andproppants, must be transported during certain oilfield well completionoperations. The reasons for such transport include, for example, to propopen fractures in a hydrocarbon recovery zone, to act as a filteringmedium to prevent formation solids from flowing to the surface, or tostabilize a formation around the wellbore at the reservoir site. Thistransport procedure involves adding a desired concentration of the solidparticles to a carrier fluid, then pumping this resultant slurry intothe desired location in the wellbore such that the solids cover theentire interval of formation.

[0005] Uniformity in packing is particularly desirable when a verticalwellbore has been drilled in an unconsolidated sand reservoir. In thesecases the transport may be intended to enable filling, or “packing”, ofthe isolated annulus area around a sand exclusion device of some type,generally a cylindrical screen with a liner. The packed annulus area,along with the sand exclusion device, serves to filter outunconsolidated sand, known as “fines”, during the hydrocarbon recoveryfrom the reservoir. The reduction or elimination of the sand from thefluid hydrocarbon being recovered is important to increasing the purityof the recovered product and to preventing damage to equipment and/orundesired clogging of the production conduit.

[0006] Proppants are used to pack the annulus around a sand exclusiondevice. Proppant is defined as a natural or synthetic particulate solidthat, when packed, exhibits an interstitial diameter that is less thanthat of the average sand particle in the unconsolidated sand formation.These materials range in composition from naturally occurring quartzsand to manufactured alumino-silicates which includes ceramics and, incases where even greater pressure resistance is needed, sinteredbauxite. These materials are also generically referred to as gravel.

[0007] In order to effect preparation of a solids pack as describedhereinabove, a packer device is used to seal the wellbore and isolatethe annulus area between the desired portion of the tool string and thewellbore at the pack site. As the gravel or proppant is pumped into theannulus area via a carrier fluid of some type, a crossover service tooldirects the fluid/solids composition from its introduction means (thework string) to the annulus area. This enables deposition of the solidsin the annulus area and subsequent removal of the circulated carrierfluid during a continuous pumping cycle.

[0008] Those skilled in the art know that to ensure the most consistentdeposition throughout the annulus area it is necessary to use anappropriate pump rate and a correspondingly appropriate carrier fluid.If the two are not appropriately selected, the result may be unevendeposition and a poor pack that cannot fully meet its assignedrequirements. In the case of a vertical wellbore, the presence ofgravity tends to mitigate the factors that can result in a non-uniformgravel or proppant deposition in the pack. It is known in the art that awide variety of carrier fluids and a wide variety of gravel loadings inthe carrier fluid can be employed, at a variety of pump rates, for thesewellbores with satisfactory results.

[0009] However, such is not the case with horizontal and certaindeviated wellbores. Some of these non-vertical wellbores must be filledby a two-step process, which differs in important aspects from theone-step, bottom up process used for vertical wellbores. In filling suchnon-vertical wellbores, the first step is to fill the bottom two-thirds,approximately, of the annulus with solids. These solids progress to theend (or “toe”) of the well by a “progressing dune” mechanism whereinsuspended solids flow over deposited solids. This first step is denotedthe “alpha wave”. Once the alpha wave has reached the toe of the well,the solids then fill the remaining empty space in the annulus until theprogressing dune once again reaches the crossover tool. This second stepis called the “beta wave”. Because of the limited annular clearances,the overall length of packed interval, and the pressure at which theformation will fracture, variables such as carrier fluid selection,gravel loading, and pump rate must be carefully determined and monitoredto ensure that the formation does not fracture and that the annulus doesnot become plugged with solids which then operate to prevent some areasof the annulus from being uniformly packed. This plugging event iscalled “bridging”. To overcome these problems, it is often effective toreduce the gravel loading while maintaining the pump rate such that thecirculating fluid pressure does not exceed the fracture pressure of theformation. While the result of this adjustment procedure tends toimprove pack consistency, the time to accomplish the deposition issignificantly increased, sometimes to unacceptably long periods.

[0010] In view of the need for preparing consistent and uniform packs ina variety of non-vertical wellbores, it would be desirable in the art todevelop a method of preparing such packs that overcomes these andrelated problems.

SUMMARY OF THE INVENTION

[0011] Accordingly, it is an object of the present invention to providea method of solids-packing for use in non-vertical, including bothhorizontal and deviated, wellbores that ensures a highly uniform packand reduces the time necessary to accomplish this task.

[0012] In carrying out this and other objects of the invention, there isprovided a method that involves, first, pumping into the wellbore a highapparent viscosity carrier fluid, as defined, that contains a relativelyhigher amount of solid particles. This first stage of pumping is carriedout until at least a major portion of the alpha wave deposition isaccomplished. Thereafter a low apparent viscosity carrier fluid, asdefined, containing a relatively lower amount of solid particles, ispumped to accomplish the remaining portion of the alpha wave deposition,if any, and also the entire beta wave deposition. Thus, a solids pack isformed. In preferred embodiments the solids pack deposition shows equalor superior uniformity with a deposition time that is significantlyshorter than is often necessary to accomplish comparable results usingheretofore known methods.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention accomplishes its objects via a novelapproach which is easily adaptable to existing equipment onsite fortransporting the solids and offers in particular a substantial reductionin the amount of time required to form the solids pack. Such timereduction translates, in the oilfield industry, into substantial costsavings. This novel approach is essentially a way to counteract many ofthe operational problems that result in the uneven or non-uniformgravel/proppant depositions encountered in horizontal and some deviatedwellbores when packed using heretofore known methods. It offers theadditional and important advantage of also enabling application of anincreased gravel or proppant concentration which reduces the timerequired to perform such deposition, and therefore reduces what isreferred to as overall “rig time”, i.e., the cost overhead expense tooperate a rig and its facilities. In the present invention, theseadvantages are achieved through specific “staged” combinations ofcarrier fluid and solids loading. It has been found that, by matchingthe identification of the carrier fluid and its specific solids loadingto each wave or portion thereof, more solids per unit of time can bepumped as compared with conventional packing methods such as waterpacking. The result is that the entire procedure can be accomplished inmuch less time, with superior results.

[0014] As used herein, “alpha wave” refers to that part of thedeposition that occurs first, from the portion of the sand exclusiondevice that is closest to the wellhead down to the toe of the wellbore.Throughout the alpha wave deposition an equilibrium dune height ismaintained. As used herein, “equilibrium dune height” refers to thephysical level of deposition at which, based upon the velocity (i.e.,pump rate) and viscosity of the carrier fluid, deposition of the solidsis approximately equal to re-suspension of the solids. Thus and incontrast, assuming that the pump rate has not changed, the “beta wave”refers to that part of the deposition that occurs second, where theequilibrium is no longer being maintained and the solids are beingdeposited again. This occurs particularly when the deposition patternmust change direction, such as occurs when deposition is completed tothe toe and is now starting on essentially the opposite end of thewellbore, that is, deposition is now proceeding back toward thewelihead. Deposition is determined to be complete when solids completelysurround the sand exclusion device as seen by a sharp pressure increaseat the surface pumps.

[0015] It is important to note that the orientation of the wellboreaffects the deposition pattern. For example, in the case of preparing agravel-pack around a sand exclusion device in a horizontal or highlydeviated wellbore (generally one oriented with its toe at an angle ofgreater than about 60 degrees from vertical), the “alpha wave” refers tothe deposition that occurs in packing the bottom two-thirds,approximately, of the annulus area, and the “beta wave” refers to thedeposition that occurs when packing the remaining one-third,approximately, of the annulus area. However, in the case of lessdeviated wellbores, for example, frequently those oriented with theirtoes at angles of less than about 60 degrees from vertical, alpha andbeta wave deposition patterns are frequently not encountered because thesolids can effectively pack by gravity alone from the toe back to the“heel”, which is the upper demarcation of the pack site, in a one stepprocess. Thus, the present invention is preferably applied to packingwellbores as to which discernible alpha and beta wave solids depositionpatterns are encountered.

[0016] The present invention employs at least two distinct carrierfluids. The first is termed herein a “high apparent viscosity carrierfluid”. The high apparent viscosity carrier fluid useful in the presentinvention preferably has an apparent viscosity of from about 5, morepreferably from about 10, to about 20, more preferably to about 12,centipoise at a shear rate of 511 second⁻¹. It is important to note thatthese apparent viscosity ranges are as represented under downholeconditions. It will be evident to the skilled artisan that, underdownhole conditions where temperatures and pressures may be muchgreater, the apparent viscosity of any given carrier fluid would beexpected to be reduced when compared with its apparent viscosity underambient conditions. Furthermore, such reduction in apparent viscositycan be predicted with relative accuracy. Therefore, it will also beclear to the skilled artisan that it is preferable to select as the highapparent viscosity carrier fluid a material having an apparent viscosityat ambient conditions which is higher than the preferred apparentviscosity range as represented under downhole conditions. As usedherein, the phrase “as represented under downhole conditions” signifiesthat the apparent viscosity is given as actually measured at thedownhole temperature and at test pressures from about 100 psig to about1000 psig, although the actual measurement location is typically, forconvenience's sake, at the earth's surface. These test pressures aregenerally lower than the actual downhole pressure encountered at thepack site, but the resulting measurements are adequately accurate forthe purposes of the present invention because the apparent viscosity ofa fluid is only weakly influenced by variations in pressure. Anyviscometer typically used in the oilfield industry, which is capable ofwithstanding the applicable (downhole) temperature and selected testpressure, can be used to measure this apparent viscosity. The apparentviscosity can also be the calculated or modeled apparent viscosity ofthe downhole fluid based on measurement of the same fluid under ambientconditions.

[0017] Essentially any carrier fluid that meets the apparent viscosityrange given hereinabove and that exhibits non-Newtonian fluid behavior(a definitional requirement for apparent viscosity) is acceptable foruse in the present invention as the high apparent viscosity carrierfluid. However, an additional caveat is that efforts should be made toensure that the selection does not unacceptably interact with theformation, equipment, or solids particles to be employed.

[0018] In a preferred embodiment of the present invention a viscoelasticsurfactant is selected as the high apparent viscosity carrier fluid. Asused herein “viscoelastic” refers to a material having both viscous andelastic characteristics. This means that the material will deform whensubjected to stress, but when the stress is removed only a fraction ofthe deformation will remain. In various embodiments this selection canbe a liquid, solution, or gel, and can be representative of a widevariety of surface-active agents including, for example, emulsifiers,dispersants, oil-wetters, water-wetters, foamers and defoamers,combinations thereof, and the like. Those skilled in the art willrecognize that the surface activity of a given molecule depends upon themolecule's structural groups. The preferred surfactants are members ofthe long-chained amine oxide and amido amine oxide families. As usedherein, the term “long-chained” refers to compounds having preferredcarbon chain lengths from 8 to 18, more preferably from 8 to 14. Suchcompounds may be either straight-chain or branched, and it is preferredthat at least some unsaturation be present. These agents offer theadditional benefits of being relatively simple to mix and non-harmful,in most cases, to producing formations. Also preferred in the presentinvention are compounds which are capable of self-assembly in solutionand form a network of rod-like micelles.

[0019] When selected as at least one component of the high apparentviscosity carrier fluid, a viscoelastic surfactant has several importantbenefits in the present invention. It is, first and foremost, aviscosifier that leaves no residue. Residue can result in undesirablepermeability reductions and voids in the solid pack.

[0020] Second, selection of a viscoelastic surfactant can reduce theimpact of friction in the system. This reduction in friction (alsoreferred to as “drag reduction”) results in easier pumping and lessenergy use. It also decreases the risk of formation fracture becausefriction pressures are decreased. The result of these effects is moreefficient solids deposition.

[0021] Finally, use of a viscoelastic surfactant can promote and improvedehydration of the pack solids. A viscoelastic surfactant solution doesnot contain any solids; therefore, the solution cannot build a filtercake as it flows through the solids-pack and is therefore less likely toplug pore throats therein.

[0022] It should be noted that it is also possible to employ as or inthe high apparent viscosity carrier fluid other well-known viscosifyingagents exhibiting an apparent viscosity (non-Newtonian) effect. Examplesof such agents can include some types of polymeric solutions, commonlyreferred to as “viscous gels”. Nonetheless, because some viscous gelshave a tendency to promote void formation and generally to result inlooser packing which may, in turn, result in inconsistent or undesirablelevels of containment of unwanted formation sands, such are generallynot preferred in the present invention.

[0023] In the present invention the high apparent viscosity carrierfluid can be a single component or a mixture of components. When morethan one component is to be used, a particularly beneficial choice is toinclude therein a proportion of a brine. “Brine” is defined herein asany saline liquid and can include solutions of chlorides, such as, forexample, calcium chloride and sodium chloride; bromides; iodides;formates; combinations thereof; and the like. While brine alone may notexhibit sufficient viscosity to meet the requirements for the highapparent viscosity carrier fluid, it does have a density higher thanthat of fresh water and is extremely economical to use, thus cutting theincurred cost when compared with many possible one-component carrierfluid selections. Brine also lacks solid particles that can damageproduction formations, and lowers the risk of undesirable formationreactions. Since it is desirable to employ a brine in as large an amountpossible in order to reduce the overall cost of the fluid, it ispreferred that the fluid comprise from about 50, more preferably fromabout 75, still more preferably from about 90, most preferably fromabout 94, to about 99 percent brine by volume, based on total volume ofthe high apparent viscosity carrier fluid. The remainder can be one ormore of the other possible high apparent viscosity carrier fluidselections, such as the viscoelastic surfactant. In a particularlypreferred embodiment, from about 1 to about 6 percent by volume of thehigh apparent viscosity carrier fluid is a viscoelastic surfactant.

[0024] In the present invention the high apparent viscosity carrierfluid also includes, as a solids load, what is referred to herein as a“relatively higher amount of solid particles.” This load is an amount ofgravel or proppant that is preferably from about 3 to about 6, morepreferably to about 4, pounds per gallon of the carrier fluid, which isreferred to as “pounds per gallon added (“ppga”)”. While a greaterconcentration of solids can be employed, a loading that is significantlyhigher than about 6 ppga may present pumping and uniformity ofdeposition difficulties that outweigh any increase in speed ofdeposition. Therefore, such higher loadings are not preferred.

[0025] As is well-known in the art, typical gravels/proppants can beselected from natural and synthetic materials, including engineeredcompositions such as resin-coated materials and pressure-resistantmaterials. Choices can also include, for example, ceramics and othersintered materials. Where solids are being used to prepare a solids-packaround a sand exclusion device, it is preferred that the averageparticle diameter of the solid particles is selected to ensure thatformation sand of undesirable particle size range is kept out of thewellbore while an appropriate level of permeability to the hydrocarbonis attained by the pack.

[0026] The second carrier fluid used in the present invention is termedherein a “low apparent viscosity carrier fluid”. This refers to acarrier fluid having an apparent viscosity ranging preferably from about0.25, more preferably from about 0.5, to less than about 5, morepreferably less than about 4, and most preferably less than about 2,centipoise at 511 second⁻¹. Because of the low apparent viscosity, amajority of these fluids will tend to exhibit Newtonian flow behavior;however, it is not required that the fluid be characteristicallyNewtonian per se. Again, the apparent viscosity range is as representedunder downhole conditions of temperature and pressure. As with the highapparent viscosity carrier fluid, it is anticipated that the exaggeratedtemperatures and pressures experienced at the downhole pack site duringthe deposition cycle will operate to reduce the apparent viscosity ofthe low apparent viscosity carrier fluid to a predictable extent.Therefore, again, it will be apparent to the skilled artisan that it isgenerally preferred to select a fluid with a higher apparent viscosityunder ambient conditions in order to meet the defined apparent viscosityrequirements under downhole conditions.

[0027] This second carrier fluid can be selected from a wide variety ofknown carrier fluids. For reasons of low cost and convenience, it ispreferred that this carrier fluid also comprise a major amount of abrine as defined hereinabove. To aid in significantly reducing theapparent viscosity of the high apparent viscosity carrier fluid used inat least the earlier portion of the alpha wave deposition, in situ inthe wellbore, a “viscosity reducer” can be included in the low apparentviscosity carrier fluid which is pumped during the later portion of thealpha wave deposition, if any, and throughout the beta wave deposition.Because the viscosity reducer “breaks”, that is, rapidly reduces, theapparent viscosity of the in situ high apparent viscosity carrier fluid,the solids pack is fixed more tightly and the high apparent viscositycarrier fluid components, and particularly any surfactant that may beincluded therein, are more effectively removed from the wellbore andformation. This viscosity reducer is preferably a solvent, morepreferably an organic solvent, which can be selected from, for example,isopropyl alcohol (C₂H₇OH), ethylene glycol monobutyl ether (“EGMBE”),acetone, combinations thereof, and the like. In general, mostshort-chained alcohols, alcohol ethers, aldehydes and ketones areeffective in this function. Such are preferably, but not necessarily,water-soluble. As used herein, the term “short-chained” refers tocompounds having 6 or fewer carbons, and includes both straight-chainand branched compounds.

[0028] It is preferred that the selected viscosity reducer be employedat a concentration of from about 1 to about 20, more preferably to about10, percent by volume, based on total volume of the carrier fluid. Wherea solvent is included in the low apparent viscosity carrier fluid, it isfurther preferred that the remainder be one or more brines, as definedhereinabove. In some instances, brine alone may efficaciously constituteall of the low apparent viscosity carrier fluid.

[0029] This low apparent viscosity carrier fluid preferably containssolid particles in a loading range of from about 0.1, preferably fromabout 1, to about 2.5 pounds per gallon added. This range is referred toherein as a “relatively lower amount of solid particles”. It ispreferred that the loading be maximized within this range in order toshorten packing time and therefore to optimize economics.

[0030] In the practice of the present invention the selected gravel orproppant, at the specified loadings, is pumped into the annulus aroundthe sand exclusion device or otherwise around the appropriate section ofthe tool string within the wellbore, using equipment and underconditions typically employed by those skilled in the art. A crossoverservice tool is generally used to separate inflowing carrier fluid fromoutflowing carrier fluid.

[0031] Uniquely, the present invention stages the pumping, such that thehigh apparent viscosity carrier fluid, with its associated relativelyhigher solids concentration, is pumped to the pack site to accomplish atleast about the first 70 percent of the alpha wave deposition, morepreferably from about 70 percent, and most preferably from about 90percent, to about 100 percent thereof. The alpha wave deposition ismeasured from volumetric calculations based on measurements taken duringthe construction of the wellbore and from solids dune heights calculatedusing mathematical relationships based on equilibrium slurry velocityabove the dune. Immediately thereafter, the second carrier fluid, i.e.,the low apparent viscosity carrier fluid, with its associated relativelylower solids amount, is pumped to the pack site to complete the alphawave deposition, if not already completed using the high apparentviscosity carrier fluid, and also to accomplish the beta wavedeposition. The completion of the beta wave deposition is determined bythe appearance of a sharp pressure increase indicated by pressuremonitoring gauges on the pumping equipment. Such monitoring devices arewell-known in the art. It is understood by those skilled in the art thatthe time period needed to accomplish each portion of the deposition ispredictable based on modeling that takes into account the pump rate(flow velocity), annulus area, solids loading, bulk density of thesolids, and characteristics of the fluid.

[0032] The result of this staged method of the present invention ispreferably a solids-pack which exhibits superior uniformity anddesirable density and permeability, with a relative absence of voids andundesirable porosity. Such pack preparation is preferably accomplishedin a time that is significantly less than that needed to prepareequivalent diameter/length solids-packs using heretofore knownsolids-pack methods that lack, in particular, the present invention'snovel staging and selection features. It is noted that the use of anorganic solvent in the second carrier fluid (the low apparent viscositycarrier fluid) is particularly helpful in obtaining optimal performancevia this invention. This is because these viscosity reducers rapidly“break” (reduce) the viscosity of the first carrier fluid (the highapparent viscosity fluid) upon contact. The result is that many or mostof the possible detrimental effects that could potentially arise fromuse of the high apparent viscosity carrier fluid during the alpha waveof the deposition, such as void formation that could result inundesirably looser packing during that phase, are subsequently reducedor eliminated when the low apparent viscosity fluid is introduced. Theinclusion of solvent thus helps to consolidate the alpha wave packingand enable rapid and comparably dense packing during the beta wavephase.

[0033] The following examples are provided to further illustrate thepresent invention and are not meant to be, nor should they be construedas being, limitative in any way of its various embodiments.

EXAMPLES Example 1

[0034] About 510 gallons of 3% KCl brine are mixed at ambienttemperature and pressure with about 10.25 gallons of a viscoelasticsurfactant to yield a solution having an apparent viscosity of about 15centipoise at 511 second⁻¹ as measured by a rotational,direct-indicating viscometer at ambient conditions. The viscoelasticsurfactant is sold under the tradename of AMOMOX APA-T by Akzo Nobel,Inc. This solution is designated as the high apparent viscosity carrierfluid.

[0035] To this high apparent viscosity carrier fluid is added about 3ppga of 20/40 U.S. mesh gravel sold under the tradename BAKER LOW FINEby Baker Oil Tools, a division of Baker Hughes Incorporated. This graveltherefore constitutes approximately 26% by weight, based on totalweight.

[0036] A second carrier fluid, designated the low apparent viscositycarrier fluid, is prepared by mixing, under ambient conditions, about168 gallons of a 3% KCl brine with a density of 8.48 pound per gallonwith about 5 gallons of ethylene glycol monobutyl ether. This fluid hasa viscosity of about 1 centipoise at 511 second⁻¹ as measured underambient conditions.

[0037] To this low apparent viscosity carrier fluid is added about 1ppga of the BAKER LOW FINE gravel described hereinabove. This graveltherefore constitutes approximately 8.7% by weight, based on totalweight of the low viscosity carrier fluid.

[0038] A simulated downhole horizontal gravel-pack of about 200 feet (61meters) in length is prepared for the purpose of sand control around asand exclusion device which has already been inserted into the simulatedwellbore using methods and means known to those skilled in the art. Theannulus area to be packed is known to be about 12 in² with a volume tobe packed of about 122 gallons; the temperature at the gravel-pack siteis ambient temperature; and the pressure is known to be about 14.7 psig.To prepare this gravel-pack, the high apparent viscosity carrier fluidis “infused” (that is, the gravel is added via a gravel infuser, whichis a screw which turns and captures the gravel and progressively movesthe gravel into a stream of liquid) with its 26% weight/weight graveland pumped at a pump rate of about 3 barrels, or 126 gallons, per minuteto the gravel-pack simulator. The pumping is continued for a period ofabout 3 minutes, which has been predetermined, via modeling based on thewellbore construction measurements of the simulated horizontal gravelpack model, to accomplish about 80 percent of the alpha wave depositionfor the horizontal gravel-pack as described.

[0039] At the end of about 3 minutes, the low apparent viscosity carrierfluid, which has been placed in a second, separate tank, is infused withits 8.7% weight/weight gravel loading and is pumped at the same rate forabout 2.5 minutes to the gravel-pack site. This has been predeterminedto complete the remaining 20% of the alpha wave deposition. The pumpingis then continued for about 3 more minutes until the beta wavedeposition is also completed. The total pumping time is about 8.5minutes. At the end of that time the carrier fluid pumping is stoppedbecause the gravel-pack has been completed.

[0040] The result is a horizontal gravel-pack exhibiting good, uniformdensity.

Example 2

[0041] The materials and methods described in Example 1 are used toestablish a horizontal gravel-pack of the same description, except thatpumping of the high apparent viscosity carrier fluid with its associatedgravel loading is continued to complete 100% of the alpha wavedeposition, and the low apparent viscosity carrier fluid with itsassociated gravel loading is subsequently pumped to accomplish the betawave deposition.

Comparative Example A

[0042] A horizontal gravel-pack as in Example 1 is prepared using thehigh apparent viscosity carrier fluid and same gravel loading as inExample 1, but no low apparent viscosity carrier fluid or associatedrelatively lower gravel loading is employed. Pumping is done at the samerate until both alpha and beta waves of deposition are completed.

[0043] The result is a horizontal gravel-pack exhibiting voids andundesirable porosity. Total pumping time needed to complete both alphawave and beta wave deposition is about 5 minutes.

Comparative Example B

[0044] A horizontal gravel-pack as in Example 1 is prepared using thelow apparent viscosity carrier fluid and same gravel loading as inExample 1 throughout both waves of the deposition cycle. No highapparent viscosity carrier fluid or associated gravel loading isemployed. Pumping is done at the same rate.

[0045] The result is a horizontal gravel-pack exhibiting voids andinconsistencies in the packing density, in part due to the occurrence ofbridging.

Comparative Example C

[0046] A horizontal gravel-pack as in Example 1 is prepared using fresh(i.e., tap) water and a gravel loading of 1 ppga weight/volume. Pumpingis done at 3 barrels per minute until both alpha and beta waves ofdeposition are completed.

[0047] The result is a horizontal gravel-pack exhibiting good, uniformdensity. Total pumping time needed to complete both alpha and beta wavesof deposition is about 12 minutes, which is 3.5 minutes longer than inExample 1.

[0048] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof, and has beendemonstrated as effective in preparing a solids-pack for use in oilfieldcompletion operations. However, it will be evident that variousmodifications and changes can be made to the steps and components usedin the method without departing from the broader spirit or scope of theinvention as set forth in the appended claims. Accordingly, thespecification is to be regarded in an illustrative rather than arestrictive sense. For example, specific combinations of brines andviscoelastic surfactants falling within the claimed parameters, but notspecifically identified or tried in particular compositions, areanticipated and expected to be within the scope of this invention.

What is claimed is:
 1. A method of solids-packing non-vertical wellborescomprising pumping into the wellbore a high apparent viscosity carrierfluid containing a relatively higher amount of solids during at leastthe earlier portion of alpha wave deposition, then pumping into thewellbore a low apparent viscosity carrier fluid containing a relativelylower amount of solids during the later portion, if any, of the alphawave deposition and during beta wave deposition, such that a solids-packis formed.
 2. The method of claim 1 wherein the high apparent viscositycarrier fluid has an apparent viscosity of from about 5 to about 20centipoise at 511 second⁻¹, as represented under downhole conditions. 3.The method of claim 2 wherein the high apparent viscosity carrier fluidhas an apparent viscosity from about 8 to about 15 centipoise at 511second⁻¹, as represented under downhole conditions.
 4. The method ofclaim 1 wherein the low apparent viscosity carrier fluid has an apparentviscosity of from about 0.25 to less than about 5 centipoise at 511second⁻¹, as represented under downhole conditions.
 5. The method ofclaim 1 wherein the relatively higher amount of solids is from about 3to about 6 pounds per gallon added.
 6. The method of claim 5 wherein therelatively higher amount of solids is from about 3 to about 4 pounds pergallon added.
 7. The method of claim 1 wherein the relatively loweramount of solids is from about 0.1 to about 2.5 pounds per gallon added.8. The method of claim 1 wherein the high apparent viscosity carrierfluid comprises a brine, a viscoelastic surfactant, or both.
 9. Themethod of claim 8 wherein the high apparent viscosity carrier fluidcomprises both a viscoelastic surfactant and a brine, and theviscoelastic surfactant is present in an amount from about 1 to about 6percent by volume based on total volume.
 10. The method of claim 1wherein the low apparent viscosity carrier fluid comprises a brine, aviscosity reducer, or both.
 11. The method of claim 10 wherein the lowapparent viscosity carrier fluid comprises both a brine and a viscosityreducer, and the viscosity reducer is present in an amount from about 1to about 20 percent by volume based on total volume.
 12. The method ofclaim 11 wherein the viscosity reducer is present in an amount fromabout 1 to about 10 percent by volume based on total volume.
 13. Themethod of claim 1 wherein the high apparent viscosity carrier fluid isused during about the earlier 70 to 100 percent of the alpha wavedeposition, and the low apparent viscosity carrier fluid is used duringabout the later 0 to 30 percent of the alpha wave deposition and duringbeta wave deposition.
 14. The method of claim 13 wherein high apparentviscosity carrier fluid is used during about the earlier 90 to 100percent of the alpha wave deposition, and the low apparent viscositycarrier fluid is used during about the later 0 to 10 percent of thealpha wave deposition and during beta wave deposition.
 15. The method ofclaim 1 wherein the solids pack is a gravel-pack surrounding a sandexclusion device or a proppant pack in a reservoir fracture zone.
 16. Amethod of solids-packing of non-vertical wellbores comprising pumpinginto the wellbore a high apparent viscosity carrier fluid, having anapparent viscosity of from about 5 to about 20 centipoise at 511second⁻¹, the high apparent viscosity carrier fluid containing arelatively higher amount of solids, during at least the earlier portionof alpha wave deposition; then pumping into the wellbore a low apparentviscosity carrier fluid, having an apparent viscosity of from about 0.25to less than about 5 centipoise at 511 second⁻¹, the low apparentviscosity carrier fluid containing a relatively lower amount of solids,during the later portion, if any, of the alpha wave deposition andduring beta wave deposition; both apparent viscosities as representedunder downhole conditions; such that a solids-pack is formed.
 17. Themethod of claim 16 wherein the high apparent viscosity carrier fluidcontains from about 3 to about 6 pounds per gallon added of solids, andthe low apparent viscosity carrier fluid contains from about 0.1 toabout 2.5 pounds per gallon added of solids.
 18. The method of claim 17wherein the high apparent viscosity carrier fluid includes a brine, aviscoelastic surfactant, or both, and the low apparent viscosity carrierfluid includes a brine, a viscosity reducer, or both.
 19. A method ofsolids-packing of non-vertical wellbores comprising pumping into thewellbore a high apparent viscosity carrier fluid, having an apparentviscosity of from about 5 to about 20 centipoise at 511 second¹, thehigh apparent viscosity carrier fluid containing from about 3 to about 6pounds per gallon added of solids, during at least the earlier portionof alpha wave deposition; then pumping into the wellbore a low apparentviscosity carrier fluid, having an apparent viscosity of from about 0.25to less than about 5 centipoise at 511 second⁻¹, the low apparentviscosity carrier fluid containing from about 0.1 to about 2.5 poundsper gallon added of solids, during the later portion, if any, of thealpha wave deposition and during beta wave deposition; both apparentviscosities as represented under downhole conditions; such that asolids-pack is formed.
 20. The method of claim 19 wherein the highapparent viscosity carrier fluid comprises a brine, a viscoelasticsurfactant, or both, and the low apparent viscosity carrier fluidcomprises a brine, a viscosity reducer, or both.